1. Field of the Invention
The present invention relates generally to automated storage libraries and specifically to a method and apparatus for slowing and stopping a robotic picker associated with such a library.
2. Description of the Prior Art
Many business and technical applications require very large databases for storing information used in connection with the applications. Database storage requirements can exceed hundreds or thousands of gigabytes of data. Often these storage requirements can not be met by disk drive storage due to cost and/or physical space limitations. Data storage for such large databases are typically stored on magnetic tape which provides the lowest storage cost per unit of data.
Tape storage, however, generally requires the longest access time among existing technologies for retrieving tile stored data. There are two primary reasons for this. First, once a tape is loaded into a tape drive, accessing the data stored on the tape is substantially slower than accessing data stored on devices such as a direct access storage device (DASD) or an optical disk. This is primarily due to the respective transport speeds of the data containing medium as well as the bandwidth attainable by the read heads present in the respective storage devices. Secondly, the time require to locate the desired tape and transport it to the tape drive for loading can bring about substantial delays. Depending on the size of the database, the totality of data can often be stored on DASD devices which do not require transport back and forth to the read head as with tape libraries.
Early magnetic tape databases required an operator, when prompted, to retrieve a tape from a shelf and load the tape into a tape drive. This procedure was not only time consuming but was also prone to human operator errors. Automated storage libraries such as that disclosed in U.S Pat. No. 5,015,139 have been developed to overcome the delays associated with loading and unloading tapes. In addition, automated storage libraries eliminate the above described human operator errors associated with handling the tapes.
The ability to place larger amounts of data on tape has derived principally from two technological advancements. First, actual cartridge size has been diminished as a result of various mechanical advances. Secondly, media properties have been improved to store more data on a given area of tape. Data can currently be written to eighteen tracks or more. It can be thus be seen that with the improvements in physical size and media technologies that have taken place over the years, it has become possible to pack more and more data into a smaller and smaller storage element such a tape cartridge. These advances in magnetic tape technology have made automated tape libraries more appealing in recent years.
Access time and reliability is improved in an automated tape library by automatically managing the storage and retrieval of tape cartridges. Operational benefits of using an automated tape library include greater reliability in tape cartridge mounts, better predictability in request-to-mount time and improved off-shift availability. Automated tape libraries include a large number of storage slots for storing library resident tape cartridges as well as one or more tape drives connected to the data processing system. They also include a robotic picker mechanism. In some cases an input/output port is provided to allow for the insertion and removal of cartridges to and from the library. The robotic picker operates on command from tile processing system to transfer a tape cartridge between a storage slot and a tape drive within seconds. In some cases it may also move a cartridge froth one storage slot to another.
The robotic picker typically includes either a picker or a robotic arm having a vision system and a gripper system. The vision system can view a label or bar code attached to a tape cartridge in order to identify the correct cartridge to be picked. The vision system may also provide feedback to the gripper system so that the gripper system can move to the correct position in order to pick the designated tape cartridge. It will be understood by one of ordinary skill in this art that such a robotic picker can be configured to access and transport a variety of data elements, tape cartridges being only one such element. As such, it will be realized that the novel aspects of this invention can be applied in any sort of automated storage library, the automated tape library being only one possible application.
In the automated tape library, the robotic picker is moved through long distances on a track which establishes a path of conveyance between the tape cartridge library and one or more tape drive units. The robotic picker is driven by a large electric motor. To accomplish the required movement, the electric motor is used to drive a gear transmission which in turn drives a metal pinion gear against a plastic coated rack gear. During normal usage, the acceleration and deceleration of the motor is controlled by an AC servo motor controller. In the event of a power failure or emergency situation, the motor may also be stopped through the use of a mechanical brake that engages automatically to stop the rotation of the motor. The translational motion of the robotic picker is stopped by forces acting between the moving pinion gear and the stationary rack.
If the combination of braking forces applied to the motor are too strong, the forces between the pinion and rack become so high that gear teeth in the rack can be broken or damaged. In the case of an emergency stop it is very important to bring the robotic picker to a complete stop in as short a time period as possible. It is, however, unacceptable to provide this feature at the cost of broken teeth.
A robotic picker and braking circuit for significantly reducing the damage which can be caused by emergency stops and/or power failures is described in a copending, commonly assigned United States Patent application bearing an IBM internal docket number of TU9-93-002. That application was filed on the same day as the subject application. In the following discussion, the TU9-93-002 circuit will be referred to as the "DBU1" (dynamic braking unit) circuit. The DBU1 circuit, while providing controlled braking in most situations, does, however, suffer from some drawbacks. There are a number of circuit properties inherent in the DBU1 circuit that can cause mechanical braking to occur at speeds that are high enough to result in damage to the robotic picker and its related components.